Semiconductor device

ABSTRACT

A semiconductor device includes: a first well provided in a semiconductor substrate; a second well provided in the semiconductor substrate, so as to be isolated from the first well; a Schottky barrier diode formed in the first well; and a PN junction diode formed in the second well, with an impurity concentration of the PN junction thereof set higher than an impurity concentration of the Schottky junction of the Schottky barrier diode, and being connected antiparallel with the Schottky barrier diode.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2013-150673, filed on Jul. 19,2013, the entire con events of which are incorporated herein byreference.

FIELD

The embodiments discussed herein are directed to a semiconductor device.

BACKGROUND

With advancement in shrinkage and higher integration of semiconductordevices, variation in the threshold voltage of transistors due tostatistical fluctuation of channel impurities has become more apparent.The threshold voltage is one of critical parameters which determineperformances of the transistors. In order to manufacture thesemiconductor devices with high performance and high reliability, it isimportant to reduce the variation in the threshold voltage due to thestatistical fluctuation of impurity.

As one technique of reducing the variation in the threshold voltage dueto the statistical fluctuation of impurity, there has been proposed atransistor structure called DDC transistor (Deeply Depleted Channeltransistor). The DDC transistor is configured by a high-concentrationchannel impurity layer having a sharp distribution of impurityconcentration, and a non-doped, epitaxially-grown silicon layer forcedthereon.

Patent Document 1: Japanese Laid-open Patent Publication No. S62-179142

Patent Document 2: Japanese Laid-open Patent Publication No. H10-335679

Patent Document 3: Japanese Laid-open Patent Publication No. 2012-174878

The transistors having the DDC structure are very effective in terms tosuppressing the variation in the threshold voltage due to thestatistical fluctuation of impurity, but cannot suppress variation inthe threshold voltage typically due to gate length which fluctuates fromchip to chip. For low voltage operation of the transistors, it isnecessary to suppress both types of variations in the threshold voltage.While the transistors having the DDC structure are effectively correctedin the inter-chip fluctuation by applying a back bias, this makes avoltage to be applied to the veil different frost a source voltage and areference voltage, so that the latch-up immunity may degrade due tonoise induced by inverted voltage.

SUMMARY

According to one aspect of embodiment, there is provided a semiconductordevice which includes: a first well provided in a semiconductorsubstrate; a second well provided in the semiconductor substrate, so asto be isolated from the first well; a Schottky barrier diode termed inthe first well; and a first PN junction diode formed in the second well,with an impurity concentration of the PW junction thereof set higherthan an impurity concentration of the Schottky junction of the Schottkybarrier diode, and being connected antiparallel with the Schottkyterrier diode.

According to another aspect of embodiment, there is provided asemiconductor device which includes: a first well provided in asemiconductor substrate; a second well provided in the semiconductorsubstrate, so as to be isolated from the first well; a Schottky barrierdiode provided in the first well; a transistor formed in the secondwell; a first signal line connected to one terminal of the Schottkybarrier diode, through which a source voltage or a reference voltage isapplied; and a second signal line connected to the other terminal of theSchottky barrier diode and the second well, through which a voltagedifferent from the source voltage and from the reference voltage isapplied.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed cut inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention.

BRIEF DESCRIPTION TO DRAWINGS

FIG. 1 and FIG. 2 are schematic cross sectional views (part 1 and part2) illustrating configurations or a semiconductor device according to afirst embodiment;

FIG. 3 is a schematic cross sectional view illustrating a configurationof a DDC transistor;

FIG. 4 to FIG. 7 are circuit diagrams (part 1 to part 4) illustratingprotection circuits of the semiconductor device according to the firstembodiment;

FIG. 8 is a schematic cross sectional view illustrating a protectioncircuit of the semiconductor device according to the first embodiment;

FIG. 9 is a graph illustrating an impurity concentration distributionproduced when a Schottky barrier diode was formed in a well of alow-voltage transistor;

FIG. 10 is a graph illustrating an impurity concentration distributionproduced when the Schottky barrier diode was formed in a well of ahigh-voltage transistor;

FIG. 11 is a graph illustrating an impurity concentration distributionproduced when a PN junction diode was formed in the well of thehigh-voltage transistor;

FIG. 12 is a graph illustrating an impurity concentration distributionproduced when the PN junction diode was formed in the well of thelow-voltage transistor;

FIG. 13 is a graph (part 1) illustrating forward I-V characteristics ofthe PN junction diode and the Schottky barrier diode;

FIG. 14 is a graph (part 1) illustrating reverse I-V characteristics ofthe Schottky barrier diode;

FIG. 15 is a graph (part 1) illustrating reverse I-V characteristics ofthe PN junction diode;

FIG. 16 to FIG. 35 are cross sectional process diagrams (part 1 to part20) illustrating a method of manufacturing the semiconductor deviceaccording to the first embodiment;

FIG. 36 is a schematic cross sectional view illustrating a configurationof a semiconductor device according to a second embodiment;

FIG. 37 is a graph (part 2) illustrating forward I-V characteristics ofthe PN junction diode and the Schottky barrier diode;

FIG. 38 is a graph (part 2) illustrating reverse I-V characteristics ofthe Schottky barrier diode;

FIG. 39 is a graph (part 2) illustrating reverse I-V characteristics ofthe PN junction diode;

FIG. 40 to FIG. 50 are cross sectional process diagrams (part 1 to part11) illustrating a method of manufacturing the semiconductor device ofthe second embodiment;

FIG. 51 is a schematic cross sectional view illustrating a configurationof a semiconductor device according to a third embodiment; and

FIG. 52 is a schematic cross sectional view illustrating a semiconductordevice according to a fourth embodiment.

DESCRIPTION OF EMBODIMENTS First Embodiment

A semiconductor device and a method of manufacturing the same accordingto a first embodiment will be explained referring to FIGS. 1 to 34.

FIG. 1 and FIG. 2 are schematic cross sectional views illustrating aconfiguration of a semiconductor device of this embodiment. FIG. 3 is aschematic cross sectional view illustrating a configuration of a DDCtransistor. FIG. 4 to FIG. 7 are circuit diagrams illustratingprotection circuits to the semiconductor device according to thisembodiment. FIG. 8 is a schematic cross sectional view illustrating aprotection circuit of the semiconductor device according to trotsembodiment. FIG. 9 is a graph illustrating an impurity concentrationdistribution produced when a Schottky barrier diode (SBD) was formed ina well of a low-voltage transistor. FIG. 10 is an impurity concentrationdistribution produced when the Schottky barrier diode (SBD) was termedin the well of the high-voltage transistor. FIG. 11 is a graphillustrating an impurity concentration distribution produced when the PNjunction diode (LRD) was formed in the well of the high-voltagetransistor. FIG. 12 is a graph illustrating an impurity concentrationdistribution produced when a PN junction diode (LRD) was formed in thewell or the low-voltage transistor. FIG. 13 to FIG. 15 are graphsillustrating I-V characteristics of the PN junction diode (LED) andSchottky barrier diode (SBD). FIG. 16 to FIG. 35 are cross sectionalprocess diagrams illustrating a method of manufacturing thesemiconductor device according to this embodiment.

First, a configuration of the semiconductor device according to thisembodiment will be explained referring to FIG. 1 to FIG. 12.

As illustrated in FIG. 1, a P-type silicon substrate 10 is provided witha DDC-NMOS transistor region 20, a DDC-PMOS transistor region 22, ahigh-voltage NMOS transistor region 24, and a high-voltage PMOStransistor region 26. Also, as illustrated in FIG. 2, to the siliconsubstrate 10, LRD regions 28 and a SBD region 30 are provided. Eachregion has an active region as demarcated by an element isolationinsulating film 56 buried in the silicon substrate 10, and apredetermined element is formed in each active region.

In the silicon substrate 10 in the DDC-NMOS transistor, region 20,formed are a P-well 36, and a buried N-well 34 provided below the bottomof the P-well 36. Around the periphery of the P-well 36, an N-well 42 isformed. The P-well 36 is thus configured as a double well surrounded bythe buried N-well 34 and the N-well 42. In the surficial portion of theP-well 36, a P-type impurity layer 38 is formed as a channel impuritylayer. While the P-well 36 and the P-type impurity layer 38 are givendifferent reference numerals in this specification, the P-type impuritylayer 38 is understood as a part of the P-well 36, so that the P-well 36and the P-type impurity layer 38 will occasionally be referred to as theP-well 36 en bloc.

Over the P-type impurity layer 36, an epitaxially-grown silicon layer 46is formed. Over the epitaxially-grown silicon layer 46, a gateinsulating film 74 is formed. Over the gate insulating film 74, a gateelectrode 76 is formed. In the epitaxially-grown silicon layer 46 andthe silicon substrate 10, on both sides of the gate electrode 76, N-typesource/drain regions 96 are formed. Over the gate electrode 76 and overthe N-type source/drain regions 36, a metal silicide film 104 is formed.

With these constituents, a DOC-NMOS transistor 106 is formed in theDDC-NMOS transistor region 20.

In the silicon substrate 10 in the DDC-PMOS transistor region 22, theN-well 42 is formed. In the surficial portion of the N-well 42, anN-type impurity layer 44 is formed as a channel imparity layer. Whilethe N-well 42 and the N-type impurity layer 44 are given differentreference minerals in this specification, the N-type impurity layer 44is understood as a part of the N-well 42, so that the N-well 42 and theP-type impurity layer 44 may occasionally be referred to as the N-well42 en bloc.

Over, the N-type impurity layer 44, the epitaxially-grown silicon layer46 is formed. Over the epitaxially-grown silicon layer 46, the gateinsulating film 74 is formed. Over the gate insulating film 74, the gateelectrode 76 is formed. In the epitaxially-grown silicon layer 46 andthe silicon substrate 10, on both sides of the gate insulating film 74,P-type source/drain regions 98 are formed. Over the gate electrode 76and the P-type source/drain regions 98, the metal silicide film 104 isformed.

With these constituents, a DDC-PMOS transistor 108 is formed in theDDC-PMOS transistor region 22.

As illustrated in FIG. 3, each of the DDC-NMOS transistor 106 and theDDC-PMOS transistor 108 has, in their channel regions 206, a thresholdvoltage controlling layer 208 which contains a high-concentrationimpurity layer, and a non-doped, epitaxially-grown layer 210 formed onthe threshold voltage controlling layer 208. The threshold voltagecontrolling layer 208 corresponds to the P-type impurity layer 38 of theDDC-NMOS transistor 106, and also to the N-type impurity layer 44 of theDDC-PMOS transistor 108. The epitaxially-grown layer 210 corresponds tothe epitaxially-grown silicon layer 46 of the DDC-NMOS transistor 106and the DDC-PMOS transistor 108. The thus-configured transistor, calledDDC transistor (Deeply Depleted Channel transistor), is very effectivein suppressing variation in the threshold voltage due to statisticalfluctuation of impurity, and is typically useful for high-speedtransistors operated at low voltage (0.9V, for example) directed tologic circuits or the like.

The reason why the DDC-NMOS transistor 106 is formed in the double wellis that the DDC-NMOS transistor 106 may also be back-biased by a voltagedifferent from the source voltage and from the reference voltage.

In the silicon substrate 10 in the high-voltage NMOS transistor region24, a P-well 60 is formed. In the surficial portion of the P-well 60, aP-type impurity layer 62 is formed. Note that the epitaxially-grownsilicon layer 46 is formed also on the silicon substrate 10 in thehigh-voltage PMOS transistor region 24. Unlike the P-type impurity layer38 and the N-type impurity layer 44, the P-type impurity layer 62 isformed in the surficial portion of a substrate configured by stackingthe epitaxially-grown silicon layer 46 on the silicon substrate 10.While the P-well 60 and the P-type impurity layer 62 are given differentreference numerals in this specification, the P-type impurity layer 62is understood as a part of the P-well 60, so that the P-well 60 and theP-type impurity layer 62 will occasionally be referred to as the P-well60 en bloc.

Over the epitaxially-grown silicon layer 46 having the P-type impuritylayer 62 formed therein, a gate insulating film 70 is formed. Over thegate insulating film 70, a gate electrode 76 is formed. In theepitaxially-grown silicon layer 46 and the silicon substrate 10, on bothsides of the gate electrode 76, N-type source/drain regions 100 areformed. Over the gate electrode 76 and the N-type source/drain regions100, the metal silicide film 104 is formed.

With these constituents, a high-voltage NMOS transistor 110 is formed inthe high-voltage NMOS transistor region 24.

In the silicon substrate 10 in the high-voltage PMOS transistor region26, an N-well 66 is formed. In the surficial portion of the N-well 66,an N-type impurity layer 68 is formed. Note that, the epitaxially-grownsilicon layer 46 is formed also on the silicon substrate 10 in thehigh-voltage PMOS transistor region 26. Like the P-type impurity layer62, the N-type impurity layer 68 is formed in the surficial portion of asubstrate configured by stacking the epitaxially-grown silicon layer 46on the silicon substrate 10. While the N-well 66 and the N-type impuritylayer 68 are given different reference numerals in this specification,the N-type impurity layer 68 is understood as a part of the N-well 66,so that the N-well 66 and the N-type impurity layer 68 will occasionallybe referred to as the N-well 66 en bloc.

Over the epitaxially-grown silicon layer 46 having the N-type impuritylayer 68 formed therein, a gate insulating film 70 is formed. Over thegate insulating film 70, a gate electrode 76 is formed. In theepitaxially-grown silicon layer 46 and the silicon substrate 10, on bothsides of the gate electrode 76, P-type source/drain regions 102 areformed. Over the gate insulating films 76 and the N-type source/drainregions 102, the metal silicide film 104 is formed.

With these constituents, a high-voltage PMOS transistor 112 is formed inthe high-voltage NMOS transistor region 26.

The high-voltage NMOS transistor 110 and the high-voltage PMOStransistor 112 are used for a circuit portion where a voltage of 3.3 VI/O, for example, which is higher than the operating voltage of the DDCtransistor, is applied. For this purpose, the gate insulating film 70 ofthe high-voltage transistor is made thicker than the gate insulatingfilm 74 of the DDC transistor.

In the silicon substrate 10 in the LRD region 28, formed are the P-well36, and the buried N-well 34 provided below the bottom of the P-well 36.Around the periphery of the P-well 36, the N-well 42 is formed. TheP-well 36 is thus configured as a double well surrounded by the buriedN-well 34 and the N-well 42. The P-well 36 is formed at the same timewith the P-well 36 of the DDC-NMOS transistor region 20.

In the P-well 36 in the LRD region 28, an active region (left in thedrawing) which serves as an electrode lead-cut portion from an anoderegion, and an active region (right in the drawing) which serves as anelectrode lead-out portion from a cathode region, are demarcated by theelement isolation insulating film 56. In the active region which servesas the electrode lead-out portion from the anode region, a P-typeimpurity layer 94 is formed as a layer for assisting contact to theP-well 36. In the active region which serves as the electrode lead-outportion from the cathode region, an N-type impurity layer 90 is formedas the cathode region.

Note that the P-type impurity layer 94 is formed at the same time withhigh-concentration portions of the P-type source/drain regions 98 of theDDC-PMOS transistor 108, and the P-type source/drain regions 102 of thehigh-voltage PMOS transistor 112. Meanwhile, the N-type imparity layer90 is formed at the same time with high-concentration portions of theN-type source/drain regions 96 of the DDC-NMOS transistor 106, and theN-type source/drain regions 100 of the high-voltage NMOS transistor 110.

Over the N-type impurity layer 90 and the P-type impurity layer 94, themetal silicide film 104 is formed.

As a consequence, in the LRD region 28, a low resistance diode (LRD) 114configured by a PN junction formed between the P-well 36 and the N-typeimpurity layer 90 is formed.

In the silicon substrate 10 in the SBD region 30, an N-well 66 isformed. The N-well 66 is formed at the same time with the N-well 66 ofthe high-voltage PMOS transistor 26. Accordingly, like the N-well 66 inthe high-voltage PMOS transistor 26, the N-well 66 has in the surficialportion thereof the N-type imparity layer 68.

In the N-well 66 in the SBD region 30, an active region (right in thedrawing) which serves as an electrode lead-out portion from tune anoderegion, and an active region (left in the drawing) which serves as anelectrode lead-out portion from the cathode region, are formed asdemarcated by the element isolation insulating film 56. In the surfaceperipheral portion of the active region which serves as the electrodelead-out portion from the anode region, the P-type impurity layer 94 isformed as a guard ring. In the surficial portion of the active regionwhich serves as the electrode lead-out portion from the cathode region,the N-type impurity layer 90 is termed as a layer for assisting contactto the N-well 66.

The P-type impurity layer 94 is formed at the same time withhigh-concentration portions of the P-type source/drain regions 98 of theDDC-PMOS transistor 108, and the P-type source/drain regions 102 of thehigh-voltage PMOS transistor 112. The N-type impurity layer 90 is formedat the same time with the N-type source/drain regions 96 of the DDC-NMOStransistor 106, and the N-type source/drain regions 100 of thehigh-voltage NMOS transistor 110.

Over the N-type impurity layer 90 and the N-type impurity layer 68, ametal silicide film 104 is formed.

As a consequence, in the SBD region 30, a Schottky barrier diode (SBD)116 configured by a Schottky junction formed between the N-type impuritylayer 68 and the metal silicide film 104 is formed.

Over the silicon substrate 10 having the transistors and the diodesformed therein, an interlayer insulating film 118 is formed. In theinterlayer insulating-film 118, buried are contact plugs 120 connectedto the individual terminals of the transistors and the diodes. To eachcontact plug 120, an interconnect 122 is connected.

As described above, the semiconductor device of this embodiment has thelow-voltage transistors with the DDC structure, the high-voltagetransistors, the PN junction diode (LRD 114), and the Schottky barrierdiode (SBD 116), all of which being mounted on the single siliconsubstrate 10.

The LRD 114 and the SBD 116 are circuit elements winch form a protectioncircuit of the semiconductor device. Typically as illustrated in FIG. 4,they are connected in a reversely parallel manner (anti-parallel)between a VDD line and a VNW line, and between a VSS line and a VPWline. The VDD line herein means a source voltage line. The VSS line is areference voltage line. VNW line is a voltage line connected to theN-well 42 of the DDC-PMOS transistor 108, directed to apply a back-biasvoltage, different from the source voltage and reference voltage, to theDDC-PMOS transistor 108. The VPW line is a voltage line connected to theP-well 36 of the DDC-NMOS transistor 106, directed to apply a back-biasvoltage, different from the source voltage and the reference voltage, tothe DDC-NMOS transistor 106.

The SBD 116 is a diode directed to prevent latch-up. The transistorswith the DDC structure are very effective in terms of suppressingvariation in the threshold voltage due to statistical fluctuation ofimpurity, but cannot suppress variation in the threshold voltage whichvaries from chip to chip. While the transistors having the DDC structureare effectively suppressed in terms of the inter-chip fluctuation in thethreshold voltage by applying the back bias, this needs a voltage to beapplied to the well different from the source voltage and the referencevoltage, so that the latch-up immunity may degrade due to noise causedby inverted voltage. Now by providing the SBD 116 between the VDD lineand the VNW line, and between the VSS line and the VPW line, thelatch-up immunity may be improved, and concurrently the powerconsumption of the transistors having the DDC structure may be reduced.

The LED 114 is a surge protection diode, and is configured as abidirectional diode by two LRDs 114 connected anti-parallel.

FIG. 5 to FIG. 7 illustrate other examples of the protection circuitconfigured by the LRD 114 and the SBD 116. The protection circuitillustrated in FIG. 5 is configured to allow the SBD 116 to also act asthe LRD 114 which has been connected in the protection circuit of FIG. 4in the same direction with the SBD 116. The SBD 116 functions as a diodefor preventing latch-up, and an anti-parallel set of the LRD 114 and theSBD 116 functions as a bidirectional diode for surge protection.

A protection circuit illustrated in FIG. 6 is configured by modifyingthe LED 114, which has been connected in the protection circuit of FIG.4 antiparallel to the SBD 116, to have a double-stage configuration. Aprotection circuit illustrated in FIG. 7 is configured by modifying theLRD 114, which has been connected in the protection circuit illustratedin FIG. 7 is configured by modifying double-stage configuration.Depending on voltage applied to the VNW line or the VPW line, only witha single-stage LRD 114, a stationary current may flow from VNW to VDD,or from VSS to VPW. Given, for example, that VPW is −0.6 V, a voltageexceeding the threshold, voltage of the LRD 114 is applied between VSSand VPW, then a stationary current flows from VSS to VPW. With thedouble-stage LRD 114, such stationary current is suppressed fromflowing.

In either case, the individual diodes are arranged in independent wellselectrically isolated from each other. For example, given the siliconsubstrate 10 is P-type, the individual diodes are arranged in N-wells,or in P-well located in N-well.

Note that the SBD 116 is not always necessarily provided to both pointsbetween the VOID line and the VNW line, and between the VSS line and theVPW line. The SBD 116 may be used only either one of them, for example,for the protection circuit between the VSS line and the VPW line.

The protection circuit illustrated in FIG. 7 may be materialized byconnecting the individual diodes as illustrated in FIG. 8.

In some cases, a Schottky barrier diode for latch-up protection ismanufactured as a discrete product, and additionally mounted on acircuit board on which a semiconductor chip is mounted. This, however,increases the number of components, and pushes up the cost. Even in somecases, latch-up still occurs despite that the Schottky barrier diode ismounted on the circuit board. The present inventors found out from ourthorough investigation that this was caused by contact failure of thesemiconductor chip. Operation of the semiconductor chip is checked in astate that the semiconductor chip is inserted into a socket formed onthe circuit board, wherein any contact failure between the socket andthe semiconductor chip may be causative of latch-up, even if theSchottky barrier diode is mounted.

The above-described problem anticipated when the Schottky barrier diodeis provided as an external part is now solved by incorporating theSchottky barrier diode into the semiconductor chip, just like thesemiconductor device of this embodiment.

In the semiconductor device of this embodiment, as described previously,the SBD 116 is formed in the N-well 66 of the high-voltage PMOStransistor 26. The reason why will be explained below.

From the viewpoint of reducing leakage current, the SBD 116 ispreferably formed by a Schottky junction between a semiconductor with arelatively low impurity concentration, and a metal (metal silicide). Onepossible configuration is to use a junction between an impurity layerwhich composes the well, and a metal. The semiconductor device of thisembodiment has a well (N-well 42) of the low-voltage (DDC) transistor,and a well (N-well 66) of the high-voltage transistor, so that the SBD116 is possibly formed between either well and a metal.

FIG. 9 is a graph illustrating a depth profile of an N-type impuritycomposing the N-well 42, measured by SIMS (Secondary Ion MassSpectrometry).

When the SBD 116 is formed in the N-well 42, as seen in FIG. 9, a highconcentration N-type impurity layer 44 is formed right under themetal-semiconductor interface. Accordingly, a depletion layer is lesslikely to extend towards the semiconductor layer, so that the electricfield is intensified at the Schottky junction, to thereby increaseleakage current under applied reverse voltage.

FIG. 10 is a graph illustrating a depth profile of an N-type imparitycomposing the N-well 66, measured by SIMS.

When the SBD 116 is formed is the N-well 66, as seen in FIG. 10, ametal-semiconductor interface (Schottky junction) is formed in a regionhaving a relatively low impurity concentration of 1×10¹⁷ cm⁻² or around.Accordingly, the depletion layer is more likely to extend towards thesemiconductor layer, and this weakens the electric field strength at theSchottky junction, and contributes to suppress the leakage current whichcould occur under applied reverse voltage.

From the above, the SBD 116 is more preferably formed in the well(N-well 66) for the high-voltage transistor, rather than in the well(N-well 42) for the low-voltage transistor.

On the other hand, in the semiconductor device of this embodiment, theLRD 114 is formed in the P-well 36 for the low-voltage PMOS transistor.The reason why will be explained below.

From the functional reason expected as a surge protection element, theLRD 114 preferably has a low voltage at which forward current rises up,and is preferably formed, from this point of view, by a PN junctionformed between semiconductors having relatively nigh concentrationvalues. One possible case is to use a PN junction between the impuritylayer composing the well, and the high-concentration impurity layercomposing the source/drain regions. The semiconductor device of thisembodiment has the well (P-well 36) of the low-voltage (DDC) transistor,and the well (P-well 60) of the high-voltage transistor, so that the LRD114 is possibly formed by a junction between either well and thesource/drain regions (N-type impurity layer 90).

FIG. 11 is a graph illustrating a depth profile of a P-type impuritycomposing the P-well 60, and the N-type impurity layer 90, measured bySIMS.

When the LRD 114 is formed in the P-well 60, as seen in FIG. 11, a PNjunction is formed in a region having a relatively low impurityconcentration of 1×10¹⁷ cm⁻² or around. Accordingly, the voltage atwhich the reverse current rises up is elevated, so that the LRD 114 isnot suitable to function as the surge protection element.

FIG. 12 is a graph illustrating a depth profile of a P-type impuritycomposing the P-well 36 and the N-type impurity layer 90, measured bySIMS.

When the LRD 114 is formed in the P-well 36, as seen in FIG. 12, a PNjunction is formed in a region having a relatively high impurityconcentration exceeding 1×10¹⁸ cm⁻². Accordingly, the voltage at whichthe reverse current rises up may be lowered.

From the above, the LRD 114 is more preferably formed in the well(P-well 36) of the low-voltage transistor, rather than in the well(P-well 60) of the high-voltage transistor.

FIG. 13 comparatively illustrates the forward characteristics of theSBDs 116, and the forward characteristics of the P⁺-N junction diodes,all formed in either of the N-wells 42, 66. In the drawing, the solidline represents the SBD (SBD in HV-NW) formed in the N-well 66 of thehigh-voltage transistor. The chain single-dashed line represents the SBD(SBD in LV-NW) formed in the N-well 42 of the low-voltage transistor.The dotted line represents the P⁺-N junction diode (P⁺-N in LV-NW)formed in the N-well 42 of the low-voltage transistor. The chaindouble-dashed line represents the P⁺-N junction diode (P⁺-N in HV-NW)formed in the N-well 66 of the high-voltage transistor. Current andvoltage values are given in absolute values.

As may be understood from the forward characteristics illustrated inFIG. 13, the SBD turns ON with a lower voltage than the P⁺-N junctiondiode does, irrespective of in which N-well the SBD was formed, so thatthe SBD can release electric charge induced by noise or the like, beforethe forward current induced by noise or the life flows through the P⁺-Njunction, diode to cause latch-up, and thereby the latch-up isavoidable.

FIG. 14 comparatively illustrates the reverse characteristics of the SBD116. In the drawing, the solid line represents the SBD (SBD in HV-NW)formed in the N-well of the high-voltage transistor. The chainsingle-dashed line represents the SBD (SBD in LV-NW) formed in theN-well of the low-voltage transistor. Current and voltage values aregiven in absolute values.

As may be understood from FIG. 14, the reverse leakage current is muchlarger in SBD 116 formed in the N-well 42 of the low-voltage transistor,than in the SBD 116 formed in the N-well 66 of the high-voltagetransistor.

It was verified from these results that, by forming the SBD 116 in theN-well 66, obtained were electrical characteristics suitable for theSchottky barrier diode for preventing latch-up, exemplified by that itcan turn ON with a low forward voltage and causes only a small reversecurrent.

FIG. 15 comparatively illustrates the reverse characteristics of the LRD114. In the drawing, the dotted line represents the LRD (LRD in LV-PW)formed in the P-well of the low-voltage transistor. The chaindouble-dashed line represents the LRD (LRD in HV-PW) formed in theP-well of the high-voltage transistor. Current and voltage values aregiven in absolute values.

As may be understood from FIG. 15, the LRD 114 when formed in the P-well68 shows only a very small voltage dependence of the reverse current, sothat current does not flow therethrough even applied with a very highvoltage, indicating that the LRD 114 cannot discharge a high surgevoltage applied, thereto. On the other hand, the LRD 114 when formed inthe P-well 36 shows a large voltage dependence of the reverse current,and a low breakdown voltage, indicating that, the LRD 114 can rapidlydischarge a high surge voltage even if applied thereto.

It was verified from these results that, by forming the LRD 114 in theP-well 36, electrical characteristics suitable for the PN junction diodeused as a surge protection element, exemplified by a low voltage atwhich forward current rises up, were obtained.

Next, a method of manufacturing the semiconductor device according tothis embodiment will be explained referring to FIG. 16 to FIG. 35. Notethat, in FIG. 16 to FIG. 35, the LRD 114 is represented only by the PNjunction portion (the right active region in FIG. 2) out from the LRDregion 28. The SBD 116 is represented only by a Schottky junctionportion (the right active region in FIG. 2) out from the SBD region 30.

First, over the F-type silicon substrate 10, a photoresist film 12 isformed by photolithography. The photoresist film 12 has an opening 14formed in a region where a trench 16, later serves as a mask alignmentmark, will be formed. The opening 14 is formed outside theproduct-forming region of the silicon substrate 10, typically in thescribe region.

Next, the silicon substrate 10, masked by the photoresist film 12, isetched in the opening 14 to form the trench 16 in the silicon substrate10 (FIG. 16).

In the method of manufacturing a semiconductor device of thisembodiment, a part of wells and channel impurity layers are formed,before the element isolation insulating film 56 is formed. The trench 16is used as a mask alignment mark used in photographic processes (forforming the wells, channel impurity layers, etc.) which take placebefore the element isolation insulating film 56 is formed.

Next, the photoresist film 12 is removed typically by ashing.

Next, over the silicon substrate 10, a silicon oxide film 18 is formed,typically by thermal oxidation, as a surface protective film for thesilicon substrate 10 (FIG. 17).

Next, a photoresist film 32 is formed by photolithography so as toexpose the DDC-NMOS transistor region 20 and the LRD region 28, and tocover the residual region. The trench 16 is used as an alignment mark inthe photolithography.

Next, ion implantation is conducted using the photoresist film 32 as amask, to thereby form the buried N-well 34, the P-well 36, and theP-type imparity layer 38 respectively into the DDC-NMOS transistorregion 20 and the LRD region 28 (FIG. 18).

The buried N-well 34 is typically formed by implanting phosphorus ion(P+) at an acceleration energy of 700 keV, and a dose of 1.5×10¹³ cm⁻².The P-well 36 is typically formed by implanting boron ion (B⁺) at anacceleration energy of 135 keV, and a dose of 1.0×10¹³ cm⁻²,respectively from four directions inclined away from the direction ofnormal line on the substrate.

The P-type impurity layer 38 is formed typically by implanting germaniumion (Ge⁺) at an acceleration energy of 30 keV and a dose of 5×10¹⁴ cm⁻²;by implanting carbon ion (C⁺) at an acceleration energy of 5 keV and adose of 5×10¹⁴ cm⁻²; by implanting boron ion at an acceleration energyof 10 keV and a dose of 1.8×10¹³ cm⁻²; and by implanting boron fluorideion (BF₂ ⁺) at an acceleration energy of 25 keV and a dose of 6×10¹²cm⁻² or at an acceleration energy of 10 keV and a dose of 2.3×10¹² cm⁻²,respectively. Germanium acts to amorphize the silicon, substrate 10 tothereby prevent channeling of boron ion, and to amorphize the siliconsubstrate 10 to thereby make a carbon atom more likely to be located ata lattice point. Carbon atom located at the lattice point acts tosuppress boron from diffusing. From this point of view, germanium ion isimplanted prior to carbon and boron. The P-well 36 is preferably formedprior to the P-type impurity layer 38.

Next, the photoresist film 32 is removed typically by ashing.

Next, a photoresist film 40 is formed by photolithography so as toexpose the DDC-PMOS transistor region 22, the DDC-NMOS transistor region20, and a region around the P-well 36 in the LRD region 28, and to coverthe residual region. The trench 16 is used as an alignment mark in thephotolithography.

Next, ion implantation is conducted using the photoresist film 40 as amask, to thereby form the P-well 42 and the N-type impurity layer 44 inthe DDC-PMOS transistor region 22 and in the region around the P-well 36(FIG. 19).

The N-well 42 is typically formed by implanting phosphorus ion at anacceleration energy or 330 keV and a dose of 7.5×10¹² cm⁻², respectivelyfrom four directions inclined away from the direction of normal line onthe substrate; and by implanting antimony ion (Sb⁺) at an accelerationenergy of 80 keV and a dose of 1.2×10¹³ cm⁻², and at an accelerationenergy of 130 keV and a dose of 6×10¹² cm⁻².

The N-type impurity layer 44 is typically formed by implanting antimonyion at an acceleration energy of 20 keV and a dose of 6×10¹² cm⁻².

In this way, the P-well 36 is now given as a double well surrounded bythe N-well 42 and the buried N-well 34. The N-well which surrounds theP-well 36 may alternatively be the N-well 66 described later.

Next, the photoresist film 40 is removed typically by ashing.

Having described an exemplary case where two kinds of DDC transistor areformed, an additional DDC transistor having a different thresholdvoltage value or a different operating voltage value may be formed byrepeating the same processes as described above, or, only by adding ionimplantation for controlling the threshold voltage, so thereby form apredetermined well and an impurity layer which serves as a channelregion.

Next, the product is annealed in an inert atmosphere, to thereby restoredamaged portions induced in the silicon substrate 10 by ionimplantation, and to activate the implanted impurities. For example, theproduct is annealed in a nitrogen atmosphere at a temperature of 600° C.for 150 seconds.

Next, the silicon oxide film 18 is removed typically by wet etchingusing an aqueous hydrofluoric acid, solution.

Next, over the surface of the silicon substrate 10, a non-doped siliconlayer (epitaxially-grown silicon layer) 46 of, for example, 25 nm thickis epitaxially grown, typically by CVD (FIG. 20).

Next, the surface of the epitaxially-grown silicon layer 46 iswet-oxidized under a reduced pressure typically by the ISSG (in-situsteam generation) process, to thereby form a silicon oxide film 48 of,for example, 3 nm thick. The annealing is typically conducted at 810° C.for 20 seconds.

Next, over the silicon oxide film 48, typically by reduced-pressure CVD,a silicon nitride film 50 of, for example, 80 nm thick is deposited. Thedeposition is typically conducted at 700° C. for 150 minutes.

Next, over the silicon nitride film 50, a photoresist film 52 is formedby photolithography so as to expose the element, isolation region. Thetrench 16 is used as an alignment mark in the photolithography.

Next, the silicon nitride film 50, the silicon oxide film 48, theepitaxially-grown silicon layer 46 and the silicon, substrate 10, maskedby the photoresist film 52, are anisotropically etched by dry etching.In this way, element isolation trenches 54 are formed in the elementisolation region of the silicon substrate 10 and the epitaxially-grownsilicon layer 46 (FIG. 21).

Next, the photoresist film 52 is removed typically by ashing.

Next, the surfaces of the epitaxially-grown silicon layer 46 and siliconsubstrate 10 are thermally oxidized, to thereby form a silicon oxidefilm of, for example, 10 nm thick, as a liner film, over the inner wallsof the element isolation trenches 54. The oxidation is typicallyconducted at 650° C.

Next, typically by high density plasma-assisted CVD, a silicon oxidefilm of, for example, 475 nm thick is deposited so as to fill up theelement isolation trenches 54.

Next, a portion of the silicon oxide film which resides on the surfaceof the silicon nitride film 50 is removed typically by CMP (ChemicalMechanical Polishing). In this way, according to the so-called STI(Shallow Trench Isolation) process, the element isolation insulatingfilms 56 is formed by the silicon oxide film filled in the elementisolation trenches 54 (FIG. 22).

Next, the element isolation insulating film 56, masked by the siliconnitride film 50, as etched to a depth of, for example, 50 nm or around,typically by wet etching using an aqueous hydrofluoric acid solution.The etching is directed to almost equalize the level of height of thesurface of the epitaxially-grown silicon layer 46 and the level ofheight of the surface of the element isolation insulating film 56, in afinished form of the semiconductor device.

Next, the silicon nitride film 40 is removed typically by wet etchingusing a hot phosphoric acid solution (FIG. 23).

Next, a photoresist film 58 is formed by photolithography, so as toexpose the high-voltage NMOS transistor region 24 and to cover theresidual region.

Next, ion implantation is conducted using the photoresist film 58 as amask, to thereby form the P-well 60 and the P-type impurity layer 62 inthe high-voltage NMOS transistor region 24 (FIG. 24).

The P-well 60 is formed, for example, by implanting boron ion at anacceleration energy of 150 keV and a dose of 7.5×10¹² cm⁻², respectivelyfrom four directions inclined away from the direction of normal line onthe substrate.

The P-type impurity layer 62 is formed, for example, by implanting boronfluoride ion at an acceleration energy of 5 keV and a dose of 3.2×10¹²cm⁻².

Next, the photoresist film 58 is removed typically by ashing.

Next, a photoresist film 64 is formed by photolithography, so as toexpose the high-voltage PMOS transistor region 26 and the SBD region 30.

Next, the iron implantation is conducted using the photoresist film 64as a mask, to thereby form the N-well 66 and the N-type impurity layer68, in the high-voltage PMOS transistor region 26 and in the SBD region30 (FIG. 25).

The N-well 66 is formed typically by implanting phosphorus ion at anacceleration energy of 360 keV and a dose of 7.5×10¹² cm⁻², respectivelyfrom four directions inclined away from the direction of normal line onthe substrate.

The N-type impurity layer 68 is formed typically by implanting arsenicion (As⁺) at an acceleration energy of 100 keV and a dose of 1.2×10¹²cm⁻².

Next, the photoresist film 64 is removed typically by ashing.

Next, the silicon oxide film 48 is removed typically by wet etchingusing an aqueous hydrofluoric acid solution.

Next, the surface of the epitaxially-grown silicon layer 46 is thermallyoxidized in a wet atmosphere, to thereby form a silicon oxide film 70 aof, for example, 7 nm thick over the surface of the epitaxially-grownsilicon layer 46 (FIG. 26). The silicon oxide film 70 a is formed, forexample, at 750° C. for 52 minutes.

Next, a photoresist film 72 is formed by photolithography, so as toexpose the DDC-NMOS transistor region 20, the DDC-PMOS transistor region22, the LRD region 28 and the SBD region 30, and to cover the residualregion.

Next, the silicon oxide film 70 a, masked by the photoresist film 72, isetched typically by wet etching using an aqueous hydrofluoric acidsolution. By the etching, silicon oxide film 70 a is removed in theDDC-NMOS transistor region 20, the DDC-PMOS transistor region 22, theLRD region 28 and the SBD region 30 (FIG. 27).

Next, the photoresist film 72 is removed typically by ashing.

Next, the product is wet-oxidized under a reduced pressure typically bythe ISSG process, typically at 810° C. for 8 seconds, followed byannealing in an NO atmosphere, for example, at 870° C. for 13 seconds.In this way, a silicon oxide film 74 a of, for example, 2 nm thick isformed, and the silicon oxide film 70 a is additionally oxidized, in theDDC-NMOS transistor region 20, the DDC-PMOS transistor region 22, theLRD region 23 and the SBD region 30.

In this way, the gate insulating film 74 composed of the silicon oxidefilm 74 a is formed in the DDC-NMOS transistor region 20 and theDDC-PMOS transistor region 22.

In the high-voltage NMOS transistor region 24 and the high-voltage PMOStransistor region 26, the gate insulating film 70, which is composed ofa silicon oxide film obtained by additionally oxidizing the siliconoxide film 70 a, is formed (FIG. 28).

Next, a non-doped polysilicon film of, for example, 100 nm thick isdeposited over the entire surface typically by reduced-pressure CVD. Thedeposition is conducted typically at 605° C.

Next, the polysilicon film is patterned by photolithography and dryetching. In this way, the gate electrodes 76 are formed respectively inthe DDC-NMOS transistor region 20, the DDC-PMOS transistor region 22,the high-voltage NMOS transistor region 24, and the high-voltage PMOStransistor region 26 (FIG. 29).

Next, an N-type impurity layer 73 which serves as an extension region istermed by photolithography and ion implantation in the DDC-NMOStransistor region 20. The N-type impurity layer 78 is formed typicallyby implanting arsenic ion at an acceleration energy of 1.5 keV said adose of 9.0×10¹⁴ cm⁻².

Again by photolithography and ion implantation, a P-type impurity layer80 which serves as an extension region is formed in the DDC-PMOStransistor region 22. The P-type impurity layer 80 is formed, forexample, by implanting boron ion at an acceleration energy of 0.5 keVand a dose of 3.2×10¹⁴ cm⁻².

Again by photolithography and ion implantation, an N-type impurity layer82 which serves as an LCD region is formed in the high-voltage NMOStransistor region 24. The N-type impurity layer 82 is formed, forexample, by implanting phosphorus ion at an acceleration energy of 35keV, and a dose of 1.0×10³ cm⁻².

Again by photolithography and ion implantation, a P-type impurity layer84 which serves as an LDD region is formed in the high-voltage PMOStransistor region 26 (FIG. 30). The P-type impurity layer 84 is formed,for example, by implanting boron ion at an acceleration energy of 0.5keV and a dose of 1.8×10¹⁴ cm⁻².

Next, a silicon oxide film of, for example, 74 nm thick is formedtypically by reduced pressure CVD. The deposition is conducted typicallyat 520° C.

Next, the silicon oxide film is anisotropically etched to thereby formsidewall insulating films 86 composed of the silicon oxide film, on thesidewalk portions of the gate electrodes 76 (FIG. 31).

Next, a photoresist film 88 is formed by photolithography, so as toexpose the DDC-NMOS transistor region 20, the high-voltage NMOStransistor region 24, the cathode region of the LED 114, and the wellcontact region of the SBD 116, and to cover the residual region. Thecathode region of the LRD 114 is the right active region in FIG. 2. Thewell contact region of the SBD 116 is the left active region in FIG. 2.

Next, ion implantation is conducted using the photoresist film 88, thegate electrode 76 and the sidewall insulating films 86 as a mask. Inthis way, the N-type impurity layer 90 is formed in the DDC-NMOStransistor region 20, the high-voltage NMOS transistor region 24, thecathode region of the LRD 114, and the well contact region of the SBD116 (FIG. 32). The N-type impurity layer 80 is formed, for example, byimplanting phosphorus ion at an acceleration energy of 8 keV and a doseof 1.2×10¹⁶ cm⁻².

The N-type impurity layer 90 of the DDC-NMOS transistor region 20 andthe high-voltage NMOS transistor region 24 serves as the highconcentration portions of the source/drain regions. The N-type impuritylayer 90 of the LRD region 28 serves as a cathode region or LRD. TheN-type imparity layer 90 or the SBD region 30 serves as a well contactlayer of SBD (see FIG. 2).

Next, the photoresist film 88 is removed typically by ashing.

Next, a photoresist film 92 is formed by photolithography, so as toexpose the DDC-PMOS transistor region 22, the high-voltage PMOStransistor region 26, the well contact region of the LRD 114, and acircumferential portion of the SBD region 30, and to cover the residualportion. The well contact region of the LRD 114 corresponds to the leftactive region in FIG. 2.

Next, ion implanted: ion is conducted using the photoresist film 92, thegate electrodes 76 and the sidewall insulating films 86 as a mask. Inthis way, a P-type impurity layer 94 is formed in the DDC-PMOStransistor region 22, the high-voltage PMOS transistor region 26, thewell contact region of the LRD 1314 and the SBD region 30 (FIG. 33). TheP-type imparity layer 94 is formed typically by implanting boron ion atan acceleration energy of 4 keV and a dose of 6.0×10¹⁵ cm⁻².

The P-type impurity layer 94 in the DDC-PMOS transistor region 22 andthe high-voltage PMOS transistor region 26 serves as high-concentrationportions of the source/drain regions. The P-type impurity layer 94 inthe LRD region 28 serves as a well contact layer of LRD (see FIG. 2).The P-type impurity layer 94 in the SBD region 30 serves as a guard ringof SBD.

Next, the photoresist film 92 is removed typically by ashing.

Next, the product is annealed within a short, time in an inertatmosphere typically at 1025° C. for 0 seconds, to thereby activate theimplanted impurities, and to allow them to diffuse in the gateelectrodes 76.

By the annealing, in the DDC-NMOS transistor region 20, the N-typesource/drain regions 96 configured by the N-type impurity layers 78, 90are formed. In the DDC-PMOS transistor region 22, the P-typesource/drain regions 98 configured by the P-type impurity layers 80, 94are formed. In the high-voltage NMOS transistor region 24, the N-typesource/drain regions 100 configured by the N-type impurity layers 82, 90are formed. In the high-voltage PMOS transistor region 26, the P-typesource/drain regions 102 configured by the P-type impurity layers 84, 94are formed.

Next, the metal silicide film 104 is selectively formed, respectivelyover the gate electrodes 76, over the N-type source/drain regions 96,100, P-type source/drain regions 98, 100, over the N-type impurity layer90 in the LRD region 28, and over the N-type impurity layer 68 in theSBD region (FIG. 34).

For example, the silicon oxide film is removed from the surface, acobalt film of 3.8 nm thick and a TiN film of 3 nm thick are deposited,annealed in a nitrogen atmosphere at 520° C. for 30 minutes, the TiNfilm and an unreacted portion of the cobalt film are removed, and theproduct is annealed in a nitrogen atmosphere at 700° C. for 30 minutes.According to such so-called SALICIDE process, the metal silicide film104 composed of a cobalt silicide film of, for example, 15.9 nm thick isformed.

In this way, the DDC-NMOS transistor 106 is formed in the DDC-NMOStransistor region 20. The DDC-PMOS transistor 108 is formed in theDDC-PMOS transistor region 22. The high-voltage PMOS transistor 110 isformed in the high-voltage NMOS transistor region 24. The high-voltagePMOS transistor 112 is formed in the high-voltage PMOS transistor region26. The LDR 114 is formed in the LRD region 23. The SBD 116 is formed inthe SBD region 30.

Next, a silicon nitride film of, for example, 50 nm thick is formed overthe entire surface by CVD, as an etching stopper film.

Next, a silicon oxide film of, for example, 500 nm thick is formed overthe silicon nitride film, typically by high density plasma-assisted CVD.

In this way, the interlayer insulating film 118, configured by a stackof the silicon nitride film and the silicon oxide film, is formed.

Next, the surface of the interlayer insulating film 118 is polished andplanerized, typically by CMP.

Next, the contact plugs 120 buried in the interlayer insulating film118, and the interconnects 122 which are connected to the contact plugs120 buried in the interlayer insulating film 118, are formed (FIG. 35).

After some necessary back end process, the semiconductor device of thisembodiment is completed.

As described above, according to this embodiment the Schottky barrierdiode for preventing latch-up is incorporated in a semiconductor chip,so that the latch-up is effectively avoidable even if the DDC transistoris back-biased. The semiconductor device of this embodiment is thereforeimproved in the reliability.

Second Embodiment

A semiconductor device and a method of manufacturing the same accordingto a second, embodiment will be explained, referring to FIG. 36 to FIG.50. Note that ail constituents, same as those of the semiconductordevice send the method of manufacturing the same in the first embodimentillustrated in FIG. 1 to FIG. 35, are given same reference numerals orsymbols, in order to avoid the explanation or to skip the detail.

FIG. 36 is a schematic cross sectional view illustrating a configurationof the semiconductor device of this embodiment. FIG. 37 to FIG. 39 aregraphs illustrating I-V characteristics of the PN junction diodes andthe Schottky barrier diodes. FIG. 40 to FIG. 50 are cross sectionalprocess diagrams illustrating the method of manufacturing thesemiconductor device of this embodiment.

First, the configuration of the semiconductor device of this embodimentwill be explained referring to FIG. 36.

While, in the first embodiment, the LRD 114 was formed in the P-well 36,and the SBD 116 was formed in the N-well 66, combination of the wellsand the LRD 114 and the SBD 116 formed therein are not limited thereto,provided that desired diode characteristics may be obtained.

The semiconductor device of this embodiment is configured similarly tothe semiconductor device of the first embodiment, except that, asillustrated in FIG. 36, the LRD 114 and the SBD 116 are respectivelyformed in the wells having conductivity types reverse so those in thefirst embodiment.

More specifically, the N-well 42 is formed in the LRD region 28. TheN-well 42 is formed at the same time with the N-well 42 is the DDC-PMOStransistor region 22.

In the N-well 42 in the LRD region 28, an active region (left in thedrawing) which serves as an electrode lead-out portion from the cathoderegion, and an active region (right in the drawing) which serves as anelectrode lead-out portion from the anode region are demarcated by theelement isolation insulating film 56. In the active region which servesas an electrode lead-out portion from, she cathode region, the N-typeimpurity layer 90 is formed as a contact layer to the N-well 42. In theactive region which serves as an electrode lead-out portion from theanode region, the P-type impurity layer 94 is formed as an anode region.

The P-type impurity layer 94 is formed at the same time with thehigh-concentration portions of the P-type source/drain regions 98 of theDDC-PMOS transistor 108, and of the P-type source/drain regions 102 ofthe high-voltage PMOS transistor 112. Meanwhile, the N-type impuritylayer 90 is formed at the same time with the high-concentration,portions of the N-type source/drain, regions 96 of the DDC-NMOStransistor 106, and of true N-type source/drain regions 94 of thehigh-voltage NMOS transistor 110.

Over the N-type impurity layer 90 and the P-type impurity layer 94, themetal silicide film 104 is formed.

As a consequence, in the LRD region 28, line LRD 114 configured by a PNjunction formed between, the P-type impurity layer 94 and the N-well 42is formed.

In the SBD region 30, formed are the P-well 60, and the buried N-well 34provided below the bottom of the P-well 60. In the circumference of theP-well 60, the N-well 66 is formed. The P-well 60 is thus configured asa double well surrounded by the buried N-well 34 and the N-well 66. TheP-well 60 is formed at the same time with the P-well 60 an thehigh-voltage NMOS transistor region 24. Accordingly, the P-well 60 hasin the surficial portion thereof the P-type impurity layer 62, like theP-well 60 of the high-voltage NMOS transistor.

In the P-well 60 in the SBD region 30, an active region (right in thedrawing) which serves as an electrode lead-out portion from the cathoderegion, and an active region (left in the drawing) which serves as anelectrode lead-out portion from the anode region are demarcated by theelement isolation insulating film 56. Around, the surficial portion ofthe active region which serves as the electrode lead-out portion of thecathode region, the N-type impurity layer 90 is formed as a guard ring.In the surficial portion of the active region which serves as anelectrode lead-out portion from the anode region, the P-type impuritylayer 34 is formed as a contact layer to the P-well 60.

The P-type impurity layer 94 is formed at the same time with thehigh-concentration portions of the P-type source/drain regions 98 of theDDC-PMOS transistor 108, and of the P-type source/drain regions 102 ofthe high-voltage PMOS transistor 112. Meanwhile, the N-type impuritylayer 90 is formed at the same time with the high-concentration portionsof the N-type source/drain regions 96 of the DDC-NMOS transistor 106,and of the N-type source/drain regions 94 of the high-voltage NMOStransistor 110.

Over the P-type impurity layer 94 and over the P-type impurity layer 62,the metal silicide film 104 is formed.

As a consequence, in the SBD region 30, the SBD 116 configured by aSchottky junction formed between the P-type impurity layer 62 and themetal silicide film 104 is formed.

Note that when the LRD 114 and the SBD 116 are formed in the P-well, theP-well is configured as a double well surrounded by an N-well, like theP-well 36 having formed therein the LRD 114 in the first embodiment, orlike the P-well 60 having formed therein the SBD 116 in this embodiment.The individual diodes are respectively formed in independent wells. Thesame will apply also to other embodiments.

Next, characteristics of the LRD 114 and the SBD 116 in thesemiconductor device of this embodiment will be explained, referring toFIG. 37 to FIG. 39.

FIG. 37 is a graph illustrating measured forward I-V characteristics ofthe SBD 116 and N⁺-P junction diode formed in the P-wells 36, 60.

In the drawing, the solid line represents the SBD (SBD in HV-PW) formedin the P-well 60 of the high-voltage transistor. The chain single-dashedline represents the SBD (SBD in LV-PW) formed in the P-well 36 of thelow-voltage transistor. The dotted line represents an N⁺-P junctiondiode (N⁺-P in LV-PW) formed in the P-well 36 of the low-voltagetransistor. The chain double-dashed line represents an N⁺-P junctiondiode (N⁺-P in HV-PW) formed in the P-well 36 of the high-voltagetransistor. Current and voltage values are given in absolute values.

As may be understood from the forward characteristics illustrated inFIG. 37, the SBD turns ON with a lower voltage than the P⁺-N junctiondiode does, irrespective of in which P-well the SBD was formed, so thatthe SBD can release electric charge induced by noise or the like, beforethe forward current induced by noise or the like flows through the P⁺-Njunction diode to cause latch-up, and thereby the latch-up is avoidable.

FIG. 38 comparatively illustrates reverse characteristics of the SBD116. In the drawing, the solid line represents the SBD (SBD in HV-PW)formed in the P-well 60 of the high-voltage transistor. The chainsingle-dashed line represents the SBD (SBD in LV-PW) formed in theP-well 36 of the low-voltage transistor. Current and voltage values aregiven in absolute values.

As seen in FIG. 38, the reverse leakage current is much larger in SBDformed in the P-well 36 of the low-voltage transistor, than in the SBDformed in the P-well 66 of the high-voltage transistor.

It was verified from these results that, by forming the SBD 116 in theP-well 60 of the high-voltage transistor, electrical characteristicssuitable for the Schottky barrier diode for preventing latch-up,exemplified by that it can turn ON with a low forward voltage and causesonly a small reverse current, may be obtained.

FIG. 39 comparatively illustrates reverse characteristics of the LRD114. In the drawing, the dotted line represents the LRD (LRD in HV-NW)formed in the N-well 66 of the high-voltage transistor. The chaindouble-dashed line represents the LRD (LRD in LV-PP) formed in theN-well 42 of the low-voltage transistor. Current and voltage values aregiven in absolute values.

As may be understood from FIG. 39, the LRD 114 when formed in the N-well66 of the high-voltage transistor shows only a very small voltagedependence of the reverse current, so that current does not flowtherethrough even applied with a very high voltage, indicating that theLRD 114 cannot discharge a high surge voltage applied thereto. On theother hand, the LRD 114 when formed in the N-well 42 of the low-voltagetransistor shows a large voltage dependence of the reverse current, anda low breakdown voltage, indicating that the LRD 114 can rapidlydischarge a high surge voltage even if applied thereto.

It was verified from these results that, by forming the LRD 114 in theN-well 42, electrical characteristics suitable for the PN junction diodeused as a surge protection element, exemplified by a low voltage atwhich forward current rises up, may be obtained.

As is clear from comparison between FIG. 14 and FIG. 38, at least forthe case where the metal electrode was composed of CoSi, the SBD 116showed better characteristics, exemplified by smaller leakage current,when formed in the N-well, rather than formed in the P-well.

Next, a method of manufacturing the semiconductor device of thisembodiment will be explained, referring to FIG. 40 to FIG. 50. Notethat, in FIG. 40 to FIG. 50, the LRD 114 is represented only by the PNjunction portion (the right active region in FIG. 36) out from the LRDregion 28. The SBD 116 is represented only by a Schottky junctionportion (the right active region in FIG. 36) out from the SBD region 30.

First, similarly to the method of manufacturing the semiconductor deviceaccording to the first embodiment illustrated in FIG. 16 and FIG. 17,the trench 16 which serves as a mask alignment mark, and the siliconoxide film 13 are formed in, and over, the P-type silicon substrate 10.

Next, a photoresist film 31 is formed by photolithography, so as toexpose the DDC-NMOS transistor region 20 and the SBD region 30, and tocover the residual region. The trench 10 is used as a mask alignmentmark in the photolithography.

Next, ion implantation is conducted using the photoresist film 31 as amask, to thereby form the buried N-well 34 in the DDC-NMOS transistorregion 20 and the SBD region 30 (FIG. 40).

Next, the photoresist film 31 is removed typically by ashing.

Next, a photoresist film 32 is formed by photolithography, so as toexpose the DDC-NMOS transistor region 20, and to cover the residualregion. The trench 16 is used as a mask alignment mark: in thephotolithography.

Next, ion implantation is conducted using the photoresist film 32 as amask, to thereby form the P-well 36 and the P-type impurity layer 38 inthe DDC-NMOS transistor region 20 (FIG. 41).

Next, the photoresist film 32 is removed typically by ashing.

Next, a photoresist film 40 is formed by photolithography, so as toexpose the DDC-PMOS transistor region 22, the LRD region 28, and aregion surrounding the P-well 36 in the DDC-NMOS transistor region 20,and to cover the residual region. The trench 16 is used as a maskalignment mark in the photolithography.

Next, ion implantation is conducted using the photoresist film 40 as amask, to thereby form the N-well 42 and the N-type impurity layer 44, inthe DDC-PMOS transistor region 22, the LRD region 28, and the regionsurrounding the P-well 36 (FIG. 42).

Now, the P-well 36 is thus configured as a double well surrounded by theburied N-well 42 and the buried N-well 34. The N-well surrounding theP-well 36 may alternatively be fine N-well 65 described later.

Next, the photoresist film 40 is removed typically by ashing.

Next, the product is annealed in an inert atmosphere, so as to restoredamaged portions induced in the silicon substrate 10 by ionimplantation, and to activate the implanted impurities. The annealing isconducted typically in a nitrogen atmosphere at 600° C. for 150 seconds.

Next, the silicon oxide film 18 is removed typically by wet etchingusing an aqueous hydrofluoric acid solution.

Next, over the surface of the silicon substrate 10, the non-dopedepitaxially-grown silicon layer 46 of, for example, 25 nm thick isformed typically by CVD (FIG. 43).

Next, similarly to the method of manufacturing the semiconductor deviceaccording to the first embodiment illustrated in FIG. 21 to FIG. 23, theelement isolation insulating film 56 which demarcates the active regionis formed in the silicon substrate 10 and the epitaxially-grown siliconlayer 46 (FIG. 44).

Next, a photoresist film 58 is formed by photolithography, so as toexpose the high-voltage NMOS transistor region 24 and the SBD region 30,and to cover the residual region.

Next, ion implantation is conducted using the photoresist film 58 as amask, to thereby form the P-well 60 and the P-type impurity layer 62both in the high-voltage NMOS transistor region 24 and the SBD region 30(FIG. 45).

Next, the photoresist film 38 is removed typically by ashing.

Next, a photoresist film 64 is formed by photolithography, so as toexpose the high-voltage PMOS transistor region 26, and a regionsurrounding the P-well 60 of the SBD region 30.

Next, ion implantation is conducted using the photoresist film 64 as amask, to thereby form the N-well 66 and the N-type impurity layer 68 inthe high-voltage PMOS transistor region 26 and around the P-well 60 inthe SBD region 30 (FIG. 46).

The P-well 60 is now configured as a double well surrounded by theN-well 66 and the buried N-well 34. The N-well which surrounds theP-well 60 may alternatively be the N-well 42 described previously.

Next, similarly to the method of manufacturing the semiconductor deviceaccording to the first embodiment illustrated in FIG. 26 to FIG. 30, thegate insulating films 70, 74, the gate electrodes 76, the N-typeimpurity layers 78, 82 and the P-type impurity layers 80, 84 are formed(FIG. 47).

Next, a silicon oxide film of, for example, 74 nm thick is formedtypically by reduced-pressure CVD. The growth, temperature is set to520° C. for example.

Next, the silicon oxide film, is anisotropically etched, to thereby formthe sidewall insulating films 86 composed of a silicon oxide film, onthe sidewall portions of the gate electrodes 76.

Next, a photoresist film 88 is formed by photolithography, so as toexpose the DDC-NMOS transistor region 20, the high-voltage NMOStransistor region 24, the cathode region of the LRD 114, and the portionaround the SBD region 30, and to cover the residual region. Now, thecathode region of the LRD 114 corresponds to the left, active region inFIG. 36.

Next, ion implantation is conducted using the photoresist film 88, thegate electrode 76 and the sidewall insulating films 36 as a mask. TheN-type impurity layers 90 are thus formed in the DDC-NMOS transistorregion 20, the high-voltage NMOS transistor region 24, the cathoderegion of the LRD 114, and the SBD region (FIG. 48).

The N-type impurity layers 90 in the DDC-NMOS transistor region 20 andin the high-voltage NMOS transistor region 24 serve as the highconcentration portions of the source/drain regions. The N-type impuritylayer 30 in the LRD region 23 serves as the cathode region of LRD. TheN-type impurity layer 90 in the SBD region 30 serves as the guard ringof SBD (see FIG. 36).

Next, the photoresist film 88 is removed typically by ashing.

Next, a photoresist film 92 is formed by photolithography, so as toexpose the DDC-PMOS transistor region 22, the high-voltage PMOStransistor region 26, the anode region of the LRD 114, and the wellcontact region of the SBD 116, and to cover the residual region. Theanode region of the LRD 114 corresponds to the right active region inFIG. 36. The well contact region of the SBD 116 corresponds to the leftactive region in FIG. 36.

Next, ion implantation is conducted using the photoresist film 92, thegate electrodes 76 and the sidewall insulating films 86 as a mask. TheP-type impurity layers 94 are thus formed in the DDC-PMOS transistorregion 22, the high-voltage PMOS transistor region 26, the anode regionof the LRD 114, and the well contact region of the SBD 116 (FIG. 49).

The P-type impurity layers 94 in the DDC-PMOS transistor region 22 andthe high-voltage PMOS transistor region 26 serve as the highconcentration portions of the source/drain regions. The P-type impuritylayer 34 in the LRD region 28 serves as the anode region of LRD. TheP-type impurity layer 94 in the SBD region 30 serves as the wellcontact, layer of SBD (see FIG. 36).

Next, the photoresist film 92 is removed typically by ashing.

Next, the product is annealed within a short time in an inert atmospheretypically at 1025° C. for 0 seconds, to thereby activate the implantedimpurities, and to allow them to diffuse in the gate electrodes 76.

By the annealing, in the DDC-NMOS transistor region 20, the N-typesource/drain regions 96 composed of the N-type impurity layers 78, 90are formed. In the DDC-PMOS transistor region 22, the P-typesource/drain regions 98 composed of the P-type impurity layers 80, 94are formed. In the high-voltage NMOS transistor region 24, the N-typesource/drain regions 100 composed of the N-type impurity layers 82, 90are formed. In the high-voltage PMOS transistor region 26, the P-typesource/drain regions 102 composed, of the P-type impurity layers 84, 04are formed.

Next, similarly to the method of manufacturing the semiconductor deviceaccording to the first embodiment illustrated in FIG. 34 and FIG. 35,the metal silicide film 104, the inter layer insulating film 118, thecontact plugs 120, the interconnects 122 and so forth are formed (FIG.50).

After some necessary back end process, the semiconductor device of thisembodiment, is completed.

As described above, according to this embodiment, the Schottky barrierdiode for preventing latch-up is incorporated in a semiconductor chip,so that the latch-up is effectively avoidable even if the DDC transistoris back-biased. The semiconductor device of this embodiment is thereforeimproved in the reliability.

Third Embodiment

A semiconductor device and a method of manufacturing the same accordingto a third embodiment will be explained, referring to FIG. 51. Note thatail constituents, same as those of the semiconductor devices and themethods of manufacturing the same in the first and second embodimentsillustrated in FIG. 1 to FIG. 50, are given same reference numerals orsymbols, in order to avoid, the explanation or to skip the detail.

FIG. 51 is a schematic cross sectional view illustrating a configurationof the semiconductor device of this embodiment.

The semiconductor device of this embodiment is configured, similarly tothe semiconductor device of the first embodiment, except that, asillustrated in FIG. 51, the LRD 114 is formed in the well having aconductivity type reverse to that in the first embodiment.

More specifically, in the LRD region 28, the N-well 42 is formed. TheN-well 42 is formed at the same time with the N-well 42 in the DDC-PMOStransistor region 22.

In the N-well 42 in the LED region 28, the active region (left, in thedrawing) which serves as an electrode lead-cut portion from the cathoderegion, and the active region (right in the drawing) which serves as anelectrode lead-out portion from the anode region are demarcated by theelement isolation insulating film 56. In the active region which servesas the electrode lead-out portion from the cathode region, the N-typeimpurity layer 90 is formed as a contact layer to the N-well 42. In theactive region which serves as the electrode lead-out portion from theanode region, the P-type impurity layer 94 is formed as the anoderegion.

Note that the P-type impurity layer 94 is formed at the same time withthe high concentration portions of the P-type source/drain regions 98 ofthe DDC-PMOS transistor 108, and of the P-type source/drain regions 102of the high-voltage PMOS transistor 112. On the other hand, the N-typeimpurity layer 90 is formed at the same time with the high concentrationportions of the N-type source/drain regions 96 of the DDC-NMOStransistor 106, and of the N-type source/drain regions 94 of thehigh-voltage NMOS transistor 110.

Over the N-type impurity layer 90 and over the P-type impurity layer 94,the metal silicide film 104 is formed.

In the LRD region 28, the LSD 114 composed of a PN junction between theP-type impurity layer 94 and the N-well 42 is thus formed.

In the SBD region 30, the N-well 66 is formed. The N-well 66 is formedat the same time with the N-well 66 of the high-voltage PMOS transistor.Accordingly, the N-well 66 has the surficial portion thereof the N-typeimpurity layer 68, like the N-well 66 of the high-voltage PMOStransistor.

In the N-well 66 of the SBD region 30, the active region which serves asthe electrode lead-out portion from the anode region (right in thedrawing), and the active region (left in the drawing) which serves as anelectrode lead-out portion from the cathode region are demarcated by theelement isolation insulating film 56. Around the surficial portion ofthe active region which serves as an electrode lead-out portion from theanode region, the P-type impurity layer 94 is formed as a guard ring. Inthe surficial portion of the active region which serves as the electrodelead-out portion from the cathode region, the N-type impurity layer 90is formed as a contact layer to the N-well 66.

The P-type impurity layer 94 is formed at the same time with the highconcentration portions of the P-type source/drain regions 98 of theDDC-PMOS transistor 108, and of the P-type source/drain regions 102 ofthe high-voltage PMOS transistor 112. On the other hand, the N-typeimpurity layer 90 is formed at the same time with the high concentrationportions of the N-type source/drain regions 96 of the DDC-PMOStransistor 106, and of the N-type source/drain regions 94 of thehigh-voltage NMOS transistor 110.

Over the N-type impurity layer 90 and over the N-type impurity layer 68,the metal silicide film 104 is formed.

As a consequence, in the SBD region 30, the SBD 116 configured by aSchottky junction formed between the N-type impurity layer 68 and themetal silicide film 104 is formed.

As has been explained in the first embodiment, the LRD 114 formed in theN-well 42 has characteristics suitable for the PN junction diode forsurge protection. On the other hand, as has been explained in the secondembodiment, the SBD 116 formed in the N-well 66 has characteristicssuitable for the Schottky diode for avoiding latch-up. Accordingly, alsoby configuring the protection circuit using the LRD 114 and the SBD 116of this embodiment, the semiconductor device with high surge resistanceand latch-up immunity may be embodied.

Characteristics and a method of manufacturing of the LRD 114 in thisembodiment are as explained in the second embodiment. Characteristicsand a method of manufacturing of the SBD 116 in this embodiment are asexplained in the first embodiment.

As described above, according to this embodiment, the Schottky barrierdiode for preventing latch-up is incorporated in a semiconductor chip,so that the latch-up is effectively avoidable even if the rid transistoris back-biased. The semiconductor device of this embodiment is thereforeimproved in the reliability.

Fourth Embodiment

A semiconductor device and a method of manufacturing the same accordingto a fourth, embodiment will be explained referring to FIG. 52. Notethat all constituents, same as those of the semiconductor devices andthe methods of manufacturing the same in the first to third embodimentsillustrated in FIG. 1 to FIG. 51, are given same reference numerals orsymbols, in order to avoid, the explanation or to skip the detail.

FIG. 52 is a schematic cross sectional view illustrating a configurationof the semiconductor device of this embodiment.

The semiconductor device of this embodiment is configured similarly tothe semiconductor device of the first embodiment, except, that, asillustrated in FIG. 52, the SBD 116 is formed in the well having aconductivity type reverse to that in the first embodiment.

More specifically, in the LRD region 28, formed are the P-well 36, andthe buried N-well 34 provided, below the bottom of the P-well 36. Aroundthe P-well 36, the N-well 42 is formed. In this way, the P-well 36 isnow configured as a double well surrounded by the buried N-well 34 andthe N-well 42. The P-well 36 is formed at the same time with the P-well36 in the DDC-NMOS transistor region 20.

In the P-well 36 in the LRD region 23, an active region (left in thedrawing) which serves as an electrode lead-out portion from, the anoderegion, and an active region (right in the drawing) which serves as anelectrode lead-out portion from the cathode region are demarcated by theelement isolation insulating film 56. In the active region which servesas the electrode lead-out portion from the anode region, the P-typeimpurity layer 94 is formed as a contact layer to the P-well 36. In theactive region which serves as the electrode lead-out portion from thecathode region, the N-type impurity layer 90 is formed as the cathoderegion.

The P-type impurity layer 94 is formed at the same time with the highconcentration portions of the P-type source/drain regions 98 of theDDC-PMOS transistor 108, and of the P-type source/drain regions 102 ofthe high-voltage PMOS transistor 112. On the other hand, the N-typeimpurity layer 90 is formed at the same time with the high concentrationportions of the N-type source/drain regions 96 of the DDC-NMOStransistor 106, and of the N-type source/drain regions 94 of thehigh-voltage NMOS transistor 110.

Over the N-type impurity layer 90 and over the P-type impurity layer 94,the metal silicide film 104 is formed.

As a consequence, in the LRD region 28, fine LRD 114 configured by a PNjunction, formed be tee en the P-well 36 and the N-type impurity layer90 is formed.

In the SBD region 30, formed are the P-well 60, and the buried N-well 34provided below the bottom of the P-well 60. Around the P-well 60, theN-well 66 is formed. In this way, the P-well 60 is now configured as adouble well surrounded by the buried N-well 34 and the N-well 66. TheP-well 60 is formed at the same time with the P-well 60 in thehigh-voltage NMOS transistor region 24. Accordingly, the P-well 60 hasin the surficial portion thereof the P-type impurity layer 62, like theP-well 60 of the high-voltage NMOS transistor.

In the P-well 60 in the SBD region 30, an active region (right in thedrawing) which serves as an electrode lead-out portion from the cathoderegion, and an active region (left in the drawing) which serves as anelectrode lead-out portion from the anode region are demarcated by theelement isolation insulating film 56. Around the surficial portion ofthe active region which serves as the electrode lead-out portion fromthe cathode region, the N-type impurity layer 30 is formed as a guardring. In the surficial portion of the active region which serves as anelectrode lead-out portion from the anode region, the P-type impuritylayer 94 is formed as a contact layer to the P-well 60.

The P-type impurity layer 94 is formed at the same time with the highconcentration portions of the P-type source/drain regions 98 of theDDC-PMOS transistor 108, and of the P-type source/drain regions 102 ofthe high-voltage PMOS transistor 112. On the other hand, the N-typeimpurity layer 90 is formed as the same time with the high concentrationportions of the N-type source/drain regions 96 of the DDC-NMOStransistor 106, and of the N-type source/drain regions 94 of thehigh-voltage NMOS transistor 110.

Over the P-type impurity layer 94 and ever the P-type impurity layer 62,the metal silicide film 104 is formed.

As a consequence, in flue SBD region 30, the SBD 116 configured by aSchottky junction formed between the P-type impurity layer 62 and themetal silicide film 104 is formed.

As has beers explained in the second embodiment, the LRD 114 formed inthe P-well 36 has characteristics suitable for the PN junction diode forsurge protection. On the other hand, as has been explained in the firstembodiment, the SBD 116 formed in the P-well 60 has characteristicssuitable for the Schottky diode for avoiding latch-up. Accordingly, alsoby configuring the protection circuit using the LRD 114 and the SBD 115of this embodiment, the semiconductor device with high surge resistanceand latch-up immunity may be embodied.

Characteristics and a method of manufacturing of the LRD 114 of thisembodiment are as explained in the first embodiment. Characteristics anda method of manufacturing of the SBD 116 of this embodiment are asexplained in the second embodiment.

As described above, according to this embodiment, the Schottky barrierdiode for preventing latch-up is incorporated in a semiconductor chip,so that the latch-up is effectively avoidable even if the DDC transistoris back-biased. The semiconductor device of this embodiment is thereforeimproved in the reliability.

Modified Embodiment

The present invention may be embodied in various ways, without limitedby the embodiments above.

For example, the guard ring provided to the Schottky junction of theSchottky barrier diodes described in the first, to fourth embodimentsexplained above, is not always necessary.

The PN junction diode for surge protection, formed according to theembodiments above in the well of the low-voltage transistor, mayalternatively be formed in the well of the high-voltage transistor.Depending on the voltage resistance relative to that of an element to beprotected, characteristics of a PN junction diode formed in the well ofthe high-voltage transistor may suffice in some cases. In these cases,both of the Schottky barrier diode and the PN junction diode may beformed in the well of the high-voltage transistor.

While the protection circuit, exemplified in the embodiments describedabove, had both of the Schottky barrier diode for avoiding latch-up andthe PN junction diode for surge protection, it is not always necessaryfor the protection circuit to have both of them, and may have onlyeither one of them.

All of configurations, constitutive materials, conditions formanufacturing and so forth of the semiconductor devices described in theembodiments above are merely illustrative, and may be modified oraltered in appropriate ways in view of common general technicalknowledge of those skilled in the art.

According to the semiconductor device disclosed herein, thesemiconductor device having a transistor with the DDC structure may beimproved in the latch-up immunity. As a consequence, the reliability ofthe semiconductor device may be improved.

All examples and conditional language provided herein are intended forthe pedagogical purposes of aiding the reader in understanding theinvention and the concepts contributed by the inventor to further theart, and are not to be construed as limitations to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority ano inferiority of the invention. Although one or more embodiments of thepresent invention have been described in detail, it should be understoodthat the various changes, substitutions, and alterations could be madehereto without departing from the spirit and scope of the invention.

What is claimed is:
 1. A semiconductor device comprising: a first wellprovided in a semiconductor substrate; a second well provided in thesemiconductor substrate, so as to be isolated from the first well; aSchottky barrier diode formed in the first well; and a first PN junctiondiode formed in the second well, with an impurity concentration of thePN junction thereof set higher than an impurity concentration of theSchottky junction of the Schottky barrier diode, acid being connectedantiparallel with the Schottky barrier diode.
 2. The semiconductordevice of claim 1, further comprising: a first signal line connected toone terminal of an antiparallel set of the Schottky barrier diode andthe first PN junction diode, and through which a source voltage or areference voltage is applied; and a second signal line connected to theother terminal, of the antiparallel set, and through which a voltagedifferent from the source voltage and from the reference voltage isapplied.
 3. The semiconductor device of claim 2, further comprising: athird well provided in the semiconductor substrate, so as to be isolatedfrom the first, well and the second well; and a first transistor formedin the third well, the second signal line being connected to the thirdwell.
 4. The semiconductor device of claim 3, wherein the semiconductorsubstrate has, in the surficial portion thereof, an epitaxially-grownsemiconductor layer, and the third well is located deeper than theepitaxially-grown semiconductor layer.
 5. The semiconductor device ofclaim 3, further comprising: a fourth well provided in the semiconductorsubstrate, so as to be isolated from the first well and the second well;a second transistor provided in the fourth well, and having a gateinsulating film, thicker than that of the first transistor.
 6. Thesemiconductor device of claim 5, wherein the semiconductor, substratehas, in the surficial portion thereof, an epitaxially-grownsemiconductor layer, and the fourth well extends from surface of theepitaxially-grown semiconductor layer.
 7. The semiconductor device ofclaim 1, wherein the semiconductor substrate has, in the surficialportion thereof, an epitaxially-grown semiconductor layer, the firstwell extends from the surface of the epitaxially-grown semiconductorlayer, and the second well is located deeper than the epitaxially-grownsemiconductor layer.
 8. The semiconductor device of claim 1, wherein theSchottky diode is formed by a Schottky junction between the first well,and an electrode formed over the first well.
 9. The semiconductor deviceof claim 1, wherein the first PN junction diode is formed by a PNjunction between the second well, and an imparity layer formed in thesurficial portion of the second well and having a conductivity typereverse to chat of the second well.
 10. The semiconductor device ofclaim 1, further comprising: a fifth well provided in the semiconductorsubstrate, so as to be isolated from the first well and the second well;and a second PN junction diode formed in the fifth well, with animpurity concentration of the PN junction thereof set higher than animpurity concentration of the Schottky junction of the Schottky barrierdiode, and being connected antiparallel with the Schottky barrier diode,a series-connected body of the first PN junction diode and the second PNjunction diode, and the Schottky barrier diode being connectedantiparallel.
 11. The semiconductor device of claim 1, furthercomprising: a sixth well provided in the semiconductor substrate, so asto be isolated from the first well and the second well, and a third PNjunction diode formed in the sixth well, with an impurity concentrationof the PN junction thereof set higher than an impurity concentration ofthe Schottky junction of the Schottky barrier diode, and being connectedantiparallel with, the Schottky barrier diode.